Here is another source about King Marko, this time from the year 1601 by Mauro Orbini, the Croatian writer. I used google translate to convert the original and fixed the wording up a little, if there is anyone who can read Croatian or Serbian and would like to have a go at translating them better, let us know.

I read somewhere that Orbini wrote that Volkasin was titled as "Rex Imperator Sclavoniae".